Cracks Found in Pickle Forks, Boeing 737 NG (Sep - Dec 2019)

[Astronuc]

Hairline cracks were detected on a crucial part of the plane called the “pickle fork,” the component that connects the wing structure, landing gear and fuselage. The NG has four pickle forks: two bracket the forward attachment frame and two bracket the rear attachment frame. The lower sections of the components are responsible for transferring most of the wing’s load into the fuselage.

The pickle fork helps manage the incredible torque, stress and aerodynamic pressures on the connection between the wings and the body. The cracks seem to have formed in the outer chord of the rear forks and the behind lying safety straps, just where they pass from the rear spar of the center wingbox to the fuselage side of the aircraft.

https://www.engineering.com/cracks-found-on-boeings-737-ng-pickle-forks/

The first cracks were found on three Quantas aircraft in September, and other airlines have reported similar cracks since then. About 50 NGs have been grounded worldwide; about 7,000 are in active service and more are in production.

The planemaker, the FAA and affected airlines don’t believe the cracks are an imminent threat to safety. While the stress of the flight cycle does eventually cause metal fatigue, aircraft can still fly with a damaged pickle fork thanks to redundant safety features such as redundant load paths, a “fail-safe” design, or a “safe life” design that minimizes stress levels. However, the pickle fork is a load-bearing component of the wing. “(Failure) may occur in turbulence or heavy landing […] and this could lead to loss of control of an aircraft,” said Steve Purvinas, federal secretary of the Australian Licensed Aircraft Engineers Association.

The structures are considered “safe life” designs, meaning they are supposed to last well beyond the life of the aircraft itself. A pickle fork is designed to withstand more than 90,000 flights worth of takeoffs and landings without cracking.

The problem is, though, that the cracks have been showing up in planes that have carried out less than a third of those projected flights. The three Quantas planes that the Australian airline grounded had only completed around 27,000 cycles. Southwest Airlines found cracks in a plane with about 28,500 cycles, and Lion Air found the damage in two planes that had flown less than 22,000 flights.

Inspections ongoing in 2020
https://www.federalregister.gov/doc...iness-directives-the-boeing-company-airplanes

I was looking for updates and any analysis as to why the pickle forks were cracking before 1/3 of design life. Still looking.

 

[gmax137]

Very interesting. The fatigue analyses I am familiar with (in structures unrelated to aircraft) were conservative, both in the transient
definitions and the specified number of cycles.
When operating experience (inspections) revealed
cracking, it was due to very localized thermal
transients occurring during very specific conditions. It would be interesting to see the assumed temperature swings in the pickle forks.

 

[Vanadium 50]

My go-to explanation is usually metallurgy, primarily because I don't understand it. But this is 7075 aluminum, which is an alloy involving eye of newt and wing of bat. Given that these micrtocracks appear in multiple places, presumably on grain boundaries, I'd start by looking at the aluminum mix, and probably more relevantly, its handling.

 

[Flyboy]

7075 is some weird stuff, as Vanadium said. One of my instructors at A&P school worked with the stuff on Lockheed L-1011s for Delta. They used it for floor stringers due to the excellent strength to weight ratios, but it turned out to be extremely picky about moisture and airflow. It would start suffering intergranular corrosion if left in an environment that was even remotely damp for extended periods of time… like under the galleys and lavatories. The rest of the floor stringers, the ones under the seats, were perfectly fine.

I’m not saying that it’s a moisture issue, but it’s definitely a material that is very picky about the environment it’s in and if it’s not exactly like the engineers assumed it would be during the design phase, it can cause problems.

 

[gmax137]

if it’s not exactly like the engineers assumed it would be during the design phase, it can cause problems.

the design engineers should know this and take the uncertainty into account when doing the fatigue analysis. Another approach would be to make sure the components are inspectable, and require those inspections.

I don't know anything about aircraft, are these pickle forks something that can be replaced? Based on @Astronuc 's reference ("the component that connects the wing structure, landing gear and fuselage") it seems that would be a major project.

 

[Vanadium 50]

Another approach would be to make sure the components are inspectable, and require those inspections.

They are and they do.

 

[Vanadium 50]

I'm still mentally hung up on the 7075.

Next time you are in a window seat, look at the wing. On the ground, it hangs below the fuselage and in the air, it's above. (As you expect) so the pickle fork is continually being flexed one way and then the other. To me, that suggests a 2000-series alloy over a 7000-series. I'm no metallurgist (and my go-to alloys are 6061 and 6063) but it seems to me you would want an alloy that has good fatigue properties and strength-to-weight would be secondary.

 

[Flyboy]

Hmmm. Some quick digging seems to point towards it having been addressed some time ago and it’s not a serious issue. Replacement appears to be not only possible but actually pretty easy, given how important of a structural member it is… 2-3 weeks to replace them.

The last info I can find on it is in 2020, and then the next thing that crops up is this thread.

 

[Astronuc]

and then the next thing that crops up is this thread.

The discovery of the cracks in 737NG pickle forks is old news, hence the dates (Sep - Dec 2019) in thread title. I looked for more recent information, but I did not see any newer articles, other than some material in the Federal Register in 2020.

I'm more interested in why it happened and the resolution to address the issue. There was some commentary about the use of winglets incorporated into the wing design, for extra lift (compared to a comparable wing without the winglets), but perhaps additional flexure of the wings, so greater stress on the root of the fork (and fatigue at or less than 1/3 of the number of cycles).

 

[Greg Bernhardt]

I was at EAA yesterday. I should have asked them in the massive hanger exhibition they had.

 

[berkeman]

I was at EAA yesterday.

Something looks a bit off in this picture from that web page...

 

[Flyboy]

Something looks a bit off in this picture from that web page...

View attachment 348871

How so?

 

[jrmichler]

Pilot step that just looks like a backwards pitot tube.

 

[berkeman]

How so?

Plane looks to have an enclosed cockpit, but a picture of a pilot seems to have been pasted onto the picture to make it look like he's in an open-cockpit plane. See how the windscreen support goes all the way around the top of the aircraft, but the Pilot is still outside the windscreen?

 

[berkeman]

Pilot step that just looks like a backwards pitot tube.

I'd wondered about that too, but I don't know much about pitot tubes, so figured maybe backwards was a "feature".

 

[Flyboy]

To answer both points…
1. That step is basically a pair of tubes welded together. Kit planes like that one are simple, and there’s no need for fancy steps. There’s only so many ways to build a streamlined pair of tubes meeting at a right angle, so a degree of convergent evolution is to be expected.
2. The frame you see there is where that bubble cockpit opens up, tipping forwards. The helmet he’s wearing is just being cautious in a confined cockpit. I would probably wear one too in that plane… I don’t want to smack my head on the canopy when it gets bumpy!

EDIT: looks like it might be a Van’s RV-12.
Prototype RV-12 pictured below.

 

[berkeman]

2. The frame you see there is where that bubble cockpit opens up, tipping forwards. The helmet he’s wearing is just being cautious in a confined cockpit. I would probably wear one too in that plane… I don’t want to smack my head on the canopy when it gets bumpy!

Sorry, I'm not buying it. His whole figure (shirt, helmet, etc.) is in *front* of the near side cockpit support. I call Photoshop...

 

[Flyboy]

Sorry, I'm not buying it. His whole figure (shirt, helmet, etc.) is in *front* of the near side cockpit support. I call Photoshop...

I’m not understanding where the problem is. That frame is behind the pilot by design. Take a look at the picture of the prototype.

Unless you mean he is closer to the camera than the frame, which I would argue is probably optical illusion.

I haven’t seen this exact airplane before, but I have seen an RV-12 and it looks pretty normal.

 

[berkeman]

Unless you mean he is closer to the camera than the frame, which I would argue is probably optical illusion.

No optical illusion, LOL. Look at the cockpit windscreen frame in your picture:

Now put a pilot in the pilot's seat and close the cockpit windscreen and you will see that near-side red windscreen support member in *front* of the pilot's shirt and helmet and face. Contrast that with my closeup of the photo where all of the pilot's image is in *front* of that blue support.

Maybe the pilot of that plane was messing around and glued an image of himself to the outside of his windscreen?

EDIT -- Upon further review, your picture seems to show a different cockpit windscreen support structure. Are you sure it's the same model of plane? There is no support member part-way back on the windscreen in your pic.

 

[Flyboy]

No optical illusion, LOL. Look at the cockpit windscreen frame in your picture:

View attachment 348893


Now put a pilot in the pilot's seat and close the cockpit windscreen and you will see that near-side red windscreen support member in *front* of the pilot's shirt and helmet and face. Contrast that with my closeup of the photo where all of the pilot's image is in *front* of that blue support.

Maybe the pilot of that plane was messing around and glued an image of himself to the outside of his windscreen?

EDIT -- Upon further review, your picture seems to show a different cockpit windscreen support structure. Are you sure it's the same model of plane? There is no support member part-way back on the windscreen in your pic.

What blue support?

As for the support frame…

I have highlighted the same structure on both images.

EDIT: upon closer inspection of both aircraft, I am convinced they are the same model. Cowling, landing gear, hinge points for the canopy, rear canopy geometry… all match up. The unpainted one is definitely a Van’s RV-12 family member. Now, it is Experimental/Amateur Built, so there’s the possibility that the builder may have made changes to their plane and the canopy frame, but most Van’s builders stay pretty close to stock on the airframe structures. Stuff like seats and interiors are where much of the customization occurs in Van’s, especially newer ones like the RV-12s.

 

[berkeman]

Hmm, you might be right. The support that goes behind the pilot may indeed be inside the cockpit.

 

[Vanadium 50]

I'm more interested in why it happened

Are aircraft maintenance records available to the public? There is an AD on this part.

This will probably take some research, but I'd start with the question "does this happen to every plane?" and if not, try and see what it correlates with: model, date, carrier, etc.

 

[Flyboy]

Are aircraft maintenance records available to the public?

No, they are not. They’re considered to be proprietary, and are only available to the owner(s), pilot(s), and mechanics.

 

[gmax137]

No, they are not. They’re considered to be proprietary, and are only available to the owner(s), pilot(s), and mechanics.

And the regulator?

In my previous life (nuclear power) the NRC got whatever they wanted. Proprietary material was marked as such, and was kept out of the public document records.

 

[Vanadium 50]

Too bad.

I am a little surprised that the FAA doesn't get them, and then FOIA comes into play.

 

[Flyboy]

And the regulator?

In my previous life (nuclear power) the NRC got whatever they wanted. Proprietary material was marked as such, and was kept out of the public document records.

My apologies. I thought it was implied they got access. I suppose I should have been more clear about that.

Generally the only times they request access is for audits or for investigation.

 

[James Demers]

The discovery of the cracks in 737NG pickle forks is old news, hence the dates (Sep - Dec 2019) in thread title. I looked for more recent information, but I did not see any newer articles, other than some material in the Federal Register in 2020.

I'm more interested in why it happened and the resolution to address the issue. There was some commentary about the use of winglets incorporated into the wing design, for extra lift (compared to a comparable wing without the winglets), but perhaps additional flexure of the wings, so greater stress on the root of the fork (and fatigue at or less than 1/3 of the number of cycles).

Here are pics of a replacement installation:
https://iainorthamerica.com/iai-replaces-the-pickle-fork-frame-fittings-for-b737-800/

Not sure what magic is used to run stringers through the new piece, but they evidently have their ways. It appears that they apply an extra coat of paint to the wheel well portion, presumably to address the environmental fussiness of the alloy.

 

[sophiecentaur]

Anyone read "No Highway" by Neville Shute?

 

[Astronuc]

Anyone read "No Highway" by Neville Shute?

I haven't read the story, but I did see the movie adaptation, No Highway in the Sky (also known as No Highway) is a 1951, starring James (Jimmy) Stewart and Marlene Dietrich
https://en.wikipedia.org/wiki/No_Highway_in_the_Sky

I always reflected on the story while studying materials behavior, especially metal fatigue, mechanical behavior or materials (including fracture toughness), and failure analysis.